A rectilinear machine is known to comprise a structure provided with at least one feeder for yarn withdrawn from a corresponding package. The yarn is carried above a machine needle bed by a carriage translating horizontally along said bed (while the needles move vertically, or perpendicular to the carriage movement direction).
During its movement, this carriage raises the needles from a rest position by knitting cams which define the needle raising and lowering “profile” (profiles predefined to enable a knitted fabric article to be produced of the required form and/or pattern). The needles hence withdraw the yarn associated with thread guides carried by the carriage, after which they are returned to their rest position in the needle bed such as to knit said yarn together with a yarn previously withdrawn and joined to other adjacent yarns in forming the article during production.
Hence by adjusting said cams (or knitting triangles), the length of each stitch can be defined, this determining the knitted yarn length.
In rectilinear machines said carriage undergoes reciprocating movements between two ends of a working path (on the rectilinear machine the carriage moving from right to left and vice versa). This movement also results in the movement of one or more thread guides, which carry the yarns used to form the stitch via said needles.
To ensure the knitting quality it is very important, in a machine of the stated type, for the yarn consumption (i.e. the yarn effectively knitted to produce the article by predefined needles) to be exactly equal during both carriage movements (from right to left and vice versa). It is therefore important to ensure that the yarn length absorbed (AYL) by the needles is constant during each complete stroke (from right to left and vice versa) of the carriage above the needle bed, this being achieved by suitably regulating said knitting cams (knitting triangles). If the AYL is not equal during the two phases of said stroke (outward towards one end and return towards the other end of the working path), the knitted yarn quantity would be different during one phase than during the other, with an evident article defect (barring).
To solve the aforesaid problem and to achieve equal AYL during the two phases of the carriage travel stroke, passive measurement devices for fed yarn are known (roller type) able to measure the effective yarn consumption in the two said phases during article production. However these devices merely measure the yarn quantity fed during each travel phase, but are unable to determine if the travel is away from or towards the yarn feeder (wherever this latter is positioned relative to the needle bed).
Constant tension yarn feed devices are also known able not only to maintain the yarn tension constant during its feed to the machine, but also to measure the yarn quantity fed to it. These devices can also determine the fed yarn quantity but not the carriage movement direction along the needle bed. Moreover all currently known devices involve lengthy adjustment times, this affecting the article time and production cost, and require to be synchronized during their operation with the various machine operating stages.
In addition, all currently known devices are devices used only during the adjustment stage, rather than the production stage; they hence provide for machine regulation within a configuration different from that normally used during the production stage.
EP 0 950 742 relates to a device for controlling the feed of yarn fed to a textile machine such as a knitting machine, hosiery machine or a bobbin winder. It comprises means for ascertaining the yarn tension and means for measuring its feed velocity to the textile machine. The ascertainment and measurement being effected by evaluation and control means which enable the tension and velocity values to be established with precision. A method implemented using the aforesaid device enables the evaluation and control means to control and regulate the operation of the textile machine and, for example, the stitch length in a knitting or hosiery machine.